Waste streams containing oil, water, high molecular weight fractions, green waxes, coke fines and other solids are produced during the refining of petroleum. These waste streams are generated as a result of the refinery process itself, cleaning processes, maintenance and other such occurrences. It is highly desireable to recover, to the extent possible, the valuable products contained in these waste streams, not only for realizing the value of the products recovered but for minimizing the amount of waste, which must be extensively treated to allow for disposal in an environmentally acceptable manner.
Normally, refinery waste streams are collected and sent to an API separator for initial processing. At the API separator, gravity separates the refinery waste into three layers, a primarily-solids or sludge layer at the bottom, a primarily-water layer in the middle and a primarily-oil layer at the top. The primarily oil layer is skimmed from the top of the API separator and collected in a large surge tank for eventual reprocessing to recover valuable products. The recovered oil or slop oil from this primarily-oil layer is typically composed of 80 to 90 weight percent oil with the remainder being water. Some entrained solids, such as green waxes and coke fines, may be present in the recovered oil but would usually be at very low levels.
Recovered oil normally contains a wide boiling range of hydrocarbon materials. Thus, reprocessing of recovered oil is typically carried out by feeding the recovered oil to a fractionator or distillation column to separate out the various products. Although other fractionators have and may be used for this reprocessing, for refineries that contain a delayed coker unit it is usually the coker fractionator that is used to fractionate the recovered oil. Conventionally, a recovered oil stream is pumped from tankage through a preheater and then fed to the bottom of the coker fractionator. However, conventional reprocessing of recovered oil has presente many operational problems.
Refinery recovered oil contains a significant amount of water. Water is immiscible in oil but is not easily separated completely from the oil in the API separator because it becomes dispersed in the oil. The dispersed water is stabilized in the oil by the high molecular weight fractions, green waxes, coke fines and other finely divided solids that are typically found entrained in recovered oil. The emulsion is not readily susceptible to emulsion breaking techniques. Although some refiners have tried using emulsion breakers in an attempt to break up the emulsion and settle out the water in a storage tank, this approach is expensive and has shown only marginal, if any, success.
When recovered oil containing a significant amount of water, such as 5 weight percent or greater, is heated through indirect heat exchange with a warmer product stream or process steam and fed to a fractionator, the water vaporizes. As the water vaporizes it expands with a significant increase in volume. The force associated with this expansion causes pressure surges which can severely damage equipment and severely upset operation of the fractionator. Such upsets can lead to contamination of the lighter product streams from the fractionator. Pressure swings in the fractionator and the reduction hydrocarbon partial pressure or stripping action caused by the water vaporization can result in heavy boiling range components being carried up he tower thereby contaminating these product streams. These contaminations then cascade into downstream process units resulting in further contaminations.
In addition, the recovered oil storage tanks must be periodically cleaned to remove the finely divided solids that have settled to the bottom of the storage tanks. The solids so removed might otherwise promote the formation of emulsion layers and are a waste that must be disposed of either by incineration, land farming or delivery to an outside waste treater. Each of these disposal methods is expensive and subject to increasingly stringent environmental controls. Yet, periodic removal is necessary with conventional recovered oil reprocessrng to keep emulsion formation and the resultant equipment damage, operational upset and product contamrnatron under control.
A process and apparatus are needed for dewatering recovered oil prior to reprocessing so that equipment damage, operational upset and product contamination are avoided. The process and apparatus should not require the use of extensive additional equipmen or complex treatment of the recovered oil so as to solve the problems associated with conventional reprocessrng with minimal capital expenditure and mlnlmal increase ln maintenance requirements. U.S. Pat. No. 4,968,407 to McGrath and Godino describes a process and apparatus for dewatering and disposing of sludge. However, the process and apparatus of U.S. Pat. No. 4,968,407 are directed to handling sludge, which typically comes from the bottom of an API separator, is composed primarily of solids and has qualities and characteristlcs much different from those of recovered oil. For example, the McGrath and Godino process feeds sludge to the top of a blowdown drum. If recovered oil containing water were fed to the top of a blowdown drum, poor water separation would be achieved and an excessive amount of oil would be carried overhead into the condensation system. Accordingly, U.S. Pat. No. 4,968,407 does not address the problems associated with dewatering recovered oil prior to reprocessing it through a fractionator.